1. Field of the Invention
The present invention relates to a communication system, a phase-locked loop, a movable body, and a communication method.
2. Description of the Related Art
In recent years, a GPS (Global Positioning System) receiver receiving a navigation message transmitted from an artificial satellite and calculating the current position thereof has been widely used by being applied to mobile telephones, car navigation systems, and the like.
Specifically, the navigation message transmitted from the artificial satellite contains orbit information indicating the orbit of the artificial satellite, and information such as transmission time of a signal. The GPS receiver receives navigation messages from four or more artificial satellites, and calculates the position of each artificial satellite from the orbit information contained in the navigation message. The GPS receiver then calculates the current three-dimensional position through a simultaneous equation on the basis of the position of each artificial satellite and the difference in the transmission time and the reception time of the navigation message. In addition, the navigation messages transmitted from the four or more artificial satellites are desired when calculating the three-dimensional position. The reason is that an error exists between the clock incorporated the GPS receiver and an atomic clock provided in the artificial satellite.
Further, each artificial satellite spreads the spectrum of data of 50 bps by using a signal termed a L1-band C/A code, that is, a pseudo-random (PRN: Pseudo-Random Noise) code, of which the code length is 1,023 and the chip rate is 1.023 MHz, and transmits the navigation message on the basis of a signal in which a carrier of 1,575.42 MHz is BPSK (Binary Phase Shift Keying) modulated by using the spread spectrum signal.
Accordingly, when the GPS receiver is intended to receive the signal transmitted from the artificial satellite, it is necessary to synchronize the PRN code, the carrier, and the data. In order to synchronize the carrier of the reception signal, a phase-locked loop (PLL: Phase-Locked Loop) is used in general GPS receivers.
The PLL has different performance in synchronization holding, that is, tracking in accordance with the loop order. For example, in the second-order PLL, phase errors occur under the condition of accelerated motion (its tracking ability under acceleration is low), but its loop stability is high (it has high resistance to noise). On the other hand, in the third-order PLL, its tracking ability under acceleration is high, but its loop stability is lower than that of the second-order PLL (it has low resistance to noise). Furthermore, international patent publication No. 99/50685 discloses an electronic navigation apparatus that includes a plurality of PLLs of different orders so as to determine parameters such as acceleration on the basis of outputs from the plurality of PLLs.